Fuel cell

ABSTRACT

Unit cell  1  comprises a pair of counterposed separators  2  and  3  and a membrane-electrode assembly (MEA) provided between the separators, MEA comprising polymeric electrolyte membrane  5  having a predetermined thickness and two reactor electrode layers  4  and  4 ′ each with a catalyst layer sandwiching the polymeric electrolyte membrane, where the polymeric electrolyte membrane and spacer sheet  5   a  having a predetermined thickness are vertically sandwiched by a pair of counterposed elastic resin gasket sheets  6  and  7 , thereby supporting MEA, and gaskets  8  and  9  of inverted V shape made from cured rubber are integrally formed on the outer surfaces of the gasket sheets, respectively, and brought into tight contact with inner surfaces  2   a  and  3   a  of separators  2  and  3 , respectively, thereby attaining desired sealing.

TECHNICAL FIELD

The present invention relates to a fuel cell, which is anelectrochemical cell capable of continuously converting chemical energygenerated between fuel and an oxide to electrical energy.

BACKGROUND ART

Generally, a fuel cell comprises a stack of several tens to severalhundred of unit cells laid one upon another to generate a large quantityof electricity, where each unit cell comprises a pair of counterposedcurrent collector electrodes (separators) and a membrane-electrodeassembly (which will be hereinafter referred to as MEA) comprising apolymeric electrolyte membrane and two reactor electrode layers eachwith a catalyst layer sandwiching the polymeric electrolyte, membrane,and MEA is provided between the separators.

It is preferable that the fuel cell has smaller dimensions particularlyin the thickness direction to make the size of the entire cell stack assmall as possible. Thus, it is desired to reduce the thickness of therespective constituent parts.

For the separators, materials with an easy current flowability such ascarbon, metal, etc. are selected, and carbon is used from the viewpointof corrosion resistance. The smaller the thickness, the better. Thus, itis desirable that the thickness is not more than about 2 mm, preferablynot more than 1 mm. Carbon separators of such a thickness have noelongation property and thus are easily breakable by excessivedeformation such as deflection, etc.

Positive or negative reactor electrode layer for use in contact with theseparator is made from anticorrosive porous carbon capable of passinghydrogen and oxygen as fuels therethrough. The reactor electrode layerhas a thickness as small as about 1 mm or less, preferably about 500 μmor less, more preferably about 300 μm or less and is also porous andthus hardly withstands deformation due to compression, etc.

Polymeric electrolyte membrane is an ion exchange membrane having athickness as small as about 1 mm or less, preferably about 500 μm orless, more preferably about 200 μm or less, which is even cross-linkedand is used in a wet state (gel state). Thus, its strength is small.

Materials for the unit cell constituent parts with such thicknesses areless elongatable and easily breakable by deformation. Thus, roughhandling during the cell assembling will give rise to breaking of theconstituent parts. Fastening of the fuel cell with a strong force toobtain tight sealing will initiate breaking from the weaker constituentparts.

For the individual unit cells thus formed, it is required to keep thedistance between the separators constant and prevent vaporization ofwater in the polymeric electrolyte membrane, thereby preventing dryingof the membrane. TI obtain the necessary sealing to prevent drying, ithas been so far proposed to use gaskets (JP-A-7-153480, JP-A-7-226220,JP-A-9-231987, etc.), or use a rubber sheet laminated with a spongelayer as a gasket (JP-A-6-96783, JP-A-7-312223, etc.), or the like.

As to the unit cell fabrication, it is desired that assembling anddisassembling of cell constituent members can be made easily, but fromthe viewpoint of a higher power generation efficiency, assembling ofunit cell by curing and fixing the constituent members with an adhesiveis a usual means somewhat at the sacrifice of assembling anddisassembling workabilities.

However, even in the case of any of the foregoing prior art proposed toobtain tight sealing to prevent drying of the polymeric electrolytemembrane, number of process steps, etc. is considerably increased,resulting in inevitable cost increase, or the resulting fuel cell couldnot always maintain satisfactory effects throughout the service life.Furthermore, the usual means of assembling the constituent members ofunit cell by curing and fixing with an adhesive can attain desiredeffects only for the initial period, but once the cell members aredeteriorated after long service, there will be a difficulty inexchanging the deteriorated members as inconvenience.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a fuel cell capable ofmaintaining a stable power generation efficiency by attaining desiredsealing of unit cells, thereby preventing drying of a polymericelectrolyte membrane, with distinguished assembling and disassemblingworkabilities, easy exchange of deteriorated constituent members, andconsiderable reduction in production cost.

The present fuel cell comprises a stack of a plurality of unit cellslaid one upon another, where the unit cell comprises a pair ofcounterposed separators, and a membrane-electrode assembly comprising apolymeric electrolyte membrane and two reactor electrode layers eachwith a catalyst layer sandwiching the polymeric electrolyte membrane,the membrane-electrode assembly being provided between the separatorsand sandwiched and supported between and by a pair of counterposed resingasket sheets, and gaskets of inverted V-shape made from cured rubberare integrally formed on outer surfaces of the gasket sheets or innersurfaces of the separators, respectively, and brought into tight contactwith the inner surfaces of the separators or the outer surfaces of thegasket sheets, respectively, thereby attaining desired sealing.

In the foregoing structure the membrane-electrode assembly can besupported by a pair of the gasket sheets, and thus themembrane-electrode assembly can be easily and exactly positioned whilekeeping the specific pressure constant in the unit cell fabrication,thereby considerably improving the working efficiency and handling inthe unit cell assembling. Desired sealing can be attained between theseparators by the gaskets and maintained stably even if the fuel cell isused as long as its service life. That is, drying due to vaporization ofwater in the polymeric electrolyte membrane can be prevented and stablepower generation efficiency can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical cross-sectional view showing the essential part ofa unit cell according to a first embodiment of the present invention.

FIG. 2 is a vertical cross-sectional view showing the essential part ofa unit cell when assembled into a fuel cell.

FIG. 3 is a vertical cross-sectional view showing the essential part ofa unit cell according to such a case that the gaskets are formed betweenpinching parts of the gasket sheets and a spacer.

FIG. 4 is a vertical cross-sectional view showing the essential part ofa unit cell according to such a case that the gaskets are formed oninner surface of the separators, respectively.

FIG. 5 is a vertical cross-sectional view showing the essential part ofa unit cell according to a second embodiment of the present invention.

FIG. 6 shows an inverted mode of positional relation between the spacerand the gaskets in the embodiment of FIG. 5.

FIG. 7 shows a mode of formation of the spacer and the gaskets on innersurfaces of the separators, respectively, in the embodiment of FIG. 6.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present fuel cell will be described in detail below,referring to drawings.

FIG. 1 is a cross-sectional view showing a first embodiment of a unitcell, which constitutes a fuel cell by stacking several tens to severalhundred of the unit cells. Unit cell 1 comprises a pair of counterposedseparators 2 and 3 of flat rectangular shape and a membrane-electrodeassembly (MEA) of likewise flat rectangular shape comprising a polymericelectrolyte membrane 5 and two reactor electrode layers 4 and 4′ eachwith a catalyst layer on surface of or within the reactor electrodelayer, the polymeric electrolyte membrane being sandwiched between thereactor electrode layers and MEA being provided between separators 2 and3.

MEA is supported by polymeric, electrolyte membrane 5 of flatrectangular shape extruded to a sufficient distance from the outerperipheral edge of MEA. Extruded peripheral part 5′ of polymericelectrolyte membrane 5 is vertically pinched and lightly supported byinner peripheral pinching parts 6′ and 7′ of a pair of counterposedelastic resin gasket sheets 6 and 7 of rectangular frame shape togetherwith spacer sheet 5 a of e.g. frame or long and narrow plate shape witha predetermined size. That is, MEA is vertically supported in a pressedstate by a pair of gasket sheets 6 and 7 through extruded peripheralparts 5′ of polymeric electrolyte membrane 5 and spacer sheet 5 a, wherethe distance between inner peripheral pinching parts 6′ and 7′, i.e. thedistance between overhanging inner peripheral edge parts of gasketsheets 6 and 7, is made smaller than the thickness of polymericelectrolyte membrane 5, thereby enabling supporting of extrudedperipheral part 5′.

Gaskets 8 and 9 of inverted V-shape cross-section molded from curedrubber are integrally formed on the outer surfaces near outer peripheraledge parts of gasket sheets 6 and 7, respectively. By bringing gaskets 8and 9 into tight contact with inner surfaces 2 a and 3 a of separators 2and 3, desired sealing can be obtained, thereby preventing vaporizationof water in the polymeric electrolyte membrane of MEA. Gaskets can bealso formed on the inner surfaces of separators, respectively.

The gaskets of inverted V-shape cross-section are made with suchdimensions as base width: about 1- about 3 mm and total height: about0.3-about 1.5 mm. To make the gasket apex into an inverted V-shape is tomakes smaller a contact area with the counter seal surface (surface tobe sealed), thereby attaining desired sealing with a smaller fasteningforce. It is preferable to set a vertical angle of the gasket apex toabout 40°-about 60° for improving the sealing. Furthermore, softmaterials with a hardness (JIS A) of 60 or less are used for thegaskets, and thus uniform and compatible sealing of the counter sealsurfaces can be attained with a smaller fastening force. Such securesealing can effectively prevent leakage of water vapor or liquids fromthe inside to the outside of the cell.

Gaskets of low-hardness materials with small base width and height arenot stable in the shape as single pieces and are very hard to handle inthe assembling and also hard to accurately provide in desired positions,and thus are integrated with gasket sheets, respectively, to reinforcethe soft gaskets and facilitate their handling.

By using the integrated members of gasket-gasket sheet, unit cells canbe fabricated stepwise. At first, two integrated members ofgasket-gasket sheet are counterposed to each other so that the gasketscan face each other, and the extruded peripheral part of a polymericelectrolyte membrane is inserted into between the inner peripheralpinching parts of the frame-shaped rectangular gasket sheets, followedby heat welding, thereby integrating the integrated members with MEA.Spacer sheet can be inserted into between the gasket sheets at the sametime. Then, the resulting integrated assembly is sandwiched between twoupper and lower separators to form a unit cell.

The unit cells thus formed are stacked together in an accurate positionwithout any damage to the reactor electrode layers, polymericelectrolyte membrane and gaskets, each with low strength.

The fuel cell of the foregoing structure according to the presentembodiment has following effects. MEA is supported by two welded gasketsheets through polymeric electrolyte membrane 5 and spacer sheet 5 a,and thus in each unit cell assembling, MEA can be easily and accuratelypositioned while keeping the specific pressure constant, therebyincreasing the workability and handling efficiency considerably in theunit cell assembling. Desired sealing can be attained between separators2 and 3 by gaskets 8 and 9 and thus the sealing can be stably maintainedtherebetween even for a long service period of fuel cell, therebypreventing drying due to vaporization of water in the polymericelectrolyte membrane and ensuring stable power generation efficiency.

Furthermore, gasket sheets 6 and 7 and spacer sheet 5 a havepredetermined thicknesses, respectively, and thus upper and lowerseparators 2 and 3 can be maintained at a constant distance therebetweeneven if the gasket sheets and the spacer sheet are sandwiched betweenthe upper and lower separators, thereby preventing MEA of fragilematerials from breaking.

FIG. 2 is a vertical cross-sectional view showing the essential part ofsealed unit cell when assembled into a fuel cell. In the unit cellassembling, inner edge parts 6 a and 7 a (corresponding to overhanginginner peripheral pinching parts 6′ and 7′, respectively) are welded orheat welded together so as to pinch extruded peripheral part 5′ of thepolymeric electrolyte membrane, and also undeformed parts of thecounterposed gasket sheets 6 b-7 b and 6 c-7 c are welded together. Forthe gasket sheets, polyester films, polyamide films, polyimide films,polyethylene naphtalate films, etc. having a thickness of about 1 mm orless, preferably about 200 μm or less, more preferably about 50 μm orless, can be used.

The present invention will be described below, referring to a specificexample.

To support MEA, polymeric electrolyte membrane 5 having a thickness of0.2 mm and stainless steel spacer sheet 5 a having a thickness of 0.7 mmwere used. Polymeric electrolyte membrane 5 was punched to a desiredshape, and the extruded peripheral part of polymeric electrolytemembrane 5 from MEA was pinched by the inner peripheral edge parts ofthe gasket sheets and heat welded together with the spacer sheet,thereby attaining integration.

As materials for forming gasket sheets 6 and 7, a polyester film (Diafoil S 100-100, product made by Mitsubishi Chemical Corp.) was used. Thefilm was punched into a frame shape of desired size.

Furthermore, gaskets 8 and 9 were integrally molded on the outersurfaces of gasket sheets 6 and 7 near the outer peripheral edge parts,respectively, by injection molding or LIM molding. As cured rubber asmaterials for molding gaskets 8 and 9, rubber with an adhesiveness tothe thermoplastic resin film can be preferably used. Any cured rubbercan be used as materials for molding gaskets 8 and 9, so long as it hasa hardness (JIAA) of 60 or less, preferably 20-40 and so long as it canattain the desired sealing.

Besides rubber of ordinary type, liquid rubber can be also used for therubber. Rubber of ordinary type for use in the gaskets includes, forexample, highly saturated type rubbers such as ethylene-propylene-basedrubber, fluororubber, hydrogenated nitrile rubber, etc., highlysaturated type thermoplastic elastomers such as hydrogenatedstyrene-isoprene copolymer, hydrogenated styrene-isoprene copolymer,etc., and so on. Liquid rubber includes, for example, liquid siliconerubber, liquid nitrile rubber, liquid ethylene-propylene-based rubber,liquid fluoro rubber, etc. The rubber is used as integrated with sheetswith a low strength, such as separators, electrodes, polymericelectrolyte membrane, etc., and thus liquid rubber moldable under lowpressure can be preferably used.

One mode of using liquid rubber as materials for molding gaskets 8 and 9is given below. Liquid silicone, rubber with a hardness of 5,000-10,000Pa·s (25° C.) before curing and a hardness (JIS A) of 40 after curing(X-34-1277 A/B, product made by Shin-Etsu Chemical Co., Ltd.) wasintegrally molded into gaskets 8 and 9 on the outer surfaces of gasketsheets 6 and 7 near the outer peripheral edge parts, respectively, byusing a mold heated at 140° C. and retaining the liquid silicone rubbertherein for 150 seconds.

FIG. 3 shows a case of gaskets 8 and 9 formed between inner peripheralpinching parts 6′ and 7′ of gasket sheets and spacer sheet 5 a, wheregasket sheets 6 and 7 and separators 2 and 3 are pressed by gaskets 8and 9 to attain sealing, and thus acid transfer from polymericelectrolyte membrane 5 towards spacer sheet 5 a can be effectivelyprevented, thereby preventing corrosion of spacer sheet 5 a by acid.

FIG. 4 shows another case of gaskets 8 and 9 formed on inner surfaces 2a and 3 a of separators 2 and 3, respectively, where corrosion of thespacer sheet by acid can be prevented.

FIG. 5 is a cross-sectional view of the essential part of unit cell 10according to a second embodiment of the present invention, where commonmembers to those of unit cell 1 according to the first embodiment of thepresent invention are identified by the same reference numerals.

In the case of unit cell 10 of the second embodiment, gasket sheets 11and 12 of different shape from that of gasket sheets 6 and 7 of thefirst embodiment are used, and spacer sheets 13 and 14 of L-shapedcross-section having predetermined dimensions are provided on the outerperipheral edge parts of gasket sheets 11 and 12, respectively. Gaskets15 and 16 are integrally molded thereon so as to fix spacer sheets 13and 14.

The embodiment will be described below, referring to a specific example.

To indirectly support MEA, polymeric electrolyte membrane 5 having athickness of 0.2 mm and stainless steel spacer sheets 13 and 14 eachhaving a thickness of 0.35 mm were punched to desired rectangular shapeand frame shape, respectively, and extruded peripheral part 5′ ofpolymeric electrolyte membrane 5 from MEA was pinched by the innerperipheral edge parts of the gasket sheets with the spacer sheets fixedby the gaskets and heat welded, thereby attaining integration.

As resin materials for forming gasket sheets 11 and 12, a polyester film(Dia foil S 100-100) was used. The film was punched into a frame shapeof desired size.

Furthermore, gaskets 15 and 16 were integrally molded on the surfaces ofgasket sheets 11 and 12 near the outer peripheral edge parts thereof,respectively, from the same maternal in the same procedure as used inthe first embodiment.

FIG. 6 shows a case where the positional relationship of spacer sheets13 and 14 and gaskets 15 and 16 is inverted. With the arrangement ofinverted positional relationship, the spacer sheets will have lesschances of exposure to acid from polymeric electrolyte membrane 5 andthus problems of spacer sheet corrosion will be hardly encountered.

FIG. 7 shows a case where spacer sheets 13 and 14 and gaskets 15 and 16in the inverted positional relationship of the case of FIG. 6 areprovided on inner surfaces 2a and 3a of separators 15 and 16,respectively.

INDUSTRIAL APPLICABILITY

In the present fuel cell, the membrane-electrode assembly (MEA) issupported by a pair of gasket sheets and thus the membrane-electrodeassembly can be easily and exactly positioned while keeping the specificpressure constant in the unit cell fabrication, thereby considerablyimproving the workability and handling efficiency in the unit cellassembling, further with distinguished assembling and disassemblingworkabilities, easy exchange of deteriorated constituent members, andconsiderable reduction in production cost as advantages.

Furthermore, desired sealing can be attained between separators bygaskets and maintained stably even if the fuel cell is used as long asits service life. That is, drying of the polymeric electrolyte membranedue to vaporization of water therein can be prevented and stable powergeneration efficiency can be obtained.

What is claimed is:
 1. A fuel cell, which comprises a stack of aplurality of unit cells laid one upon another, the unit cell comprisinga pair of counterposed separators and a membrane-electrode assemblycomprising a polymeric electrolyte membrane and two reactor electrodelayers each with a catalyst layer sandwiching the polymeric electrolytemembrane, the membrane-electrode assembly being provided between theseparators, characterized in that the membrane-electrode assembly issandwiched and supported by a pair of counterposed resin gasket sheetsholding a spacer sheet having a predetermined thickness, and gaskets ofinverted V-shape made from cured rubber, are formed on outer surfaces ofthe gasket sheets or inner surfaces of the separators, respectively, andbrought into tight contact with the inner surfaces of the separators orthe outer surfaces of the gasket sheets, respectively, thereby attainingsealing.
 2. A fuel cell according to claim 1, wherein the polymericelectrolyte membrane having a predetermined thickness is extrudedoutwardly from the peripheral edge of the membrane-electrode assembly,and the extruded peripheral part of the membrane-electrode assembly ispinched by a pair of the counterposed gasket sheets, thereby supportingthe membrane-electrode assembly, while maintaining the separators at apredetermined distance therebetween.
 3. A fuel cell according to claim2, wherein the extruded peripheral part of the polymeric electrolytemembrane is pinched by overhanging inner pinching edge parts of a pairof the counterposed gasket sheets.
 4. A fuel cell according to claim 1,wherein the spacer sheet is provided between the inner peripheralpinching edge parts of the gasket sheets and the gaskets.
 5. A fuel cellaccording to claim 1, wherein the gaskets are formed between the innerperipheral pinching edge parts of the gasket sheets and the spacersheet.
 6. A fuel cell according to claim 1, wherein the gaskets areintegrated with the gasket sheets, respectively.
 7. A fuel cellaccording to claim 1, wherein the gaskets are integrally formed togetherwith spacers, respectively.
 8. A fuel cell according to claim 1, whereinthe gaskets are made from cured rubber having a hardness (JIS A) of 60or less.
 9. A fuel cell according to claim 8, wherein the cured rubberis a cured product of liquid silicone rubber, liquid nitrile rubber,liquid ethylene-propylene-based rubber or liquid fluororubber.